Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626.
In various lighting applications, light from one or more LEDs or other light sources are mixed to provide a combined lighting effect, such as a desired chromaticity of combined light. To this end, light from each of the light sources can be controlled with regard to factors such as intensity of light. For example, instantaneous or time-average intensity of light from light sources such as LEDs can be controlled using methods such as direct drive current control and drive current pulse width modulation (PWM) control.
Controlling aspects of light from a light source such as an LED by controlling the drive signals supplied thereto can present some challenges. For example, due to factors such as device aging, device heating and ambient lighting conditions, relationships between drive signals supplied to a light source and characteristics of the light emitted in response to said drive signals can change over time. To compensate for such changes, several optical feedback solutions have been considered which measure light source input-output characteristics in mixed-light applications in order to accurately control the light emitted by each light source and thus to control the mixed light.
One solution focusing on measuring light from component light sources contemplates a plurality of light filters or filtered sensors in order to discriminate light from each light source on the basis of the spectra of light emitted thereby. Light output from each LED can be measured and compared to a desired output, and lighting corrections can be made accordingly. A drawback of this solution is that it can be costly and difficult to provide multiple color filters tuned to the light output of each LED, while rejecting the light output of other LEDs.
Another solution employs a single sensor and measures light output of different LEDs by employing an electronic control circuit which turns off the LEDs not being measured in a sequence of time pulses. This allows direct measurement of each LED independently. The measured light output for each LED is compared to a desired output, which may be determined by user inputs, and corrections to the current for each color are made accordingly. A drawback of this solution is that time intervals must be set aside for the measurement operation, which can interrupt continuity of lighting applications.
A similar solution employs a single sensor and measures light output of different LEDs by employing an electronic control circuit which turns off the LED being measured in a sequence of time pulses. The light output of the LED being measured is then computed by subtracting the light output corresponding to all LEDs but the LED being measured being on from the light output corresponding to all LEDs being on. Measured light outputs for the colors are compared to desired outputs, which may be set by user controls, and changes to the power supply for the color blocks are made as necessary. A drawback of this solution is that time intervals must be set aside for the measurement operation, which again can interrupt continuity of lighting applications.
A solution which avoids the need for specific calibration periods is implementable when PWM drive current control is used to control light from multiple LEDs, more specifically when the PWM drive pulses for each LED are partially overlapping. According to this solution, the peak light output and the drive current of a first LED are simultaneously measured at a point in time when the PWM drive pulses do not overlap, and the combined peak light output and the drive current of a second LED are simultaneously measured at another point in time when the PWM drive pulses overlap. The peak light output of the second LED is determined by subtracting the two measurements and the ratio of peak light output to peak current can be used for feedback control purposes. A drawback of this solution is that it requires monitoring of the drive currents, and there is no method provided by which the required partial overlapping of PWM drive pulses can be achieved, nor is there a method provided for initiating measurements of the light at the appropriate points in time.
Thus, there is a need in the art to provide method and apparatus by which aspects of mixed light can be controlled and measured which does not suffer from at least one of the drawbacks identified above.